The numerous parts of the central nervous system (CNS) are highly interdependent during ontogeny. Consequently, functionally significant effects of a developmental aberration at one locus may be cascaded to distant parts of the nervous system. Specifically, early lesions of the developing CNS can result in the formation of permanent, novel connections between various neural structures. Such connections can contribute to a sparing of neural function that would normally be lost following comparable lesions in the mature CNS, or the novel connections can be dysfunctional. Certain lesions of the developing visual system in hamsters and other species induce the formation of novel retinal projections to the auditory or somatosensory systems. We have used these novel neural circuits to study the mechanisms controlling the development of various features of the mammalian visual system. We also demonstrated that the anatomical and physiological organization of the novel neural circuits resemble, in many of their essential features, those of the normal visual system. Most interestingly, we showed that single neurons in the somatosensory cortex of neonatally operated hamsters with novel retinal projections to the somatosensory system have visual properties that qualitatively and quantitatively resemble those of neurons in the visual cortex of normal hamsters. These data raise two non-exclusive hypotheses - that thalamic nuclei or cortical areas at corresponding levels in different primary sensory systems process their inputs in similar ways or that during some developmental period, these structures are, to some extent, equipotential and differentiate as a function of their inputs. Our results suggest that the novel neural pathways may be able to mediate certain aspects of visually guided behavior, i.e., that the hamsters can "see" with them. In the present studies, we propose: a) to test the function of the novel pathways at the behavioral level, b) to identify some output pathways by which the novel circuits might have their behavioral effects and c) to study the development of these output pathways. Our findings are significant for mental health, neurological disease and developmental neurology in that they contribute 1) to understanding the neural mechanisms of perception and cognition, 2) to understanding the functional organization of sensory cortices and thalamic nuclei, 3) to understanding normal and pathological neural development and 4) to the development of paradigms for the surgical repair of the brain.